udf: support 2048-byte spacing of VRS descriptors on 4K media

Some UDF creators (specifically Microsoft, but perhaps others) mishandle
the ECMA-167 corner case that requires descriptors within a Volume
Recognition Sequence to be placed at 4096-byte intervals on media where
the block size is 4K. Instead, the descriptors are placed at the 2048-
byte interval mandated for media with smaller blocks. This nonconformity
currently prevents Linux from recognizing the filesystem as UDF.

Modify the driver to tolerate a misformatted VRS on 4K media.

[JK: Simplified descriptor checking]
Signed-off-by: Steven J. Magnani <steve@digidescorp.com>
Tested-by: Steven J. Magnani <steve@digidescorp.com>
Link: https://lore.kernel.org/r/20190711133852.16887-2-steve@digidescorp.com
Signed-off-by: Jan Kara <jack@suse.cz>

1 files changed, 16 insertions, 0 deletions

diff --git a/fs/udf/super.c b/fs/udf/super.c
index 14a91955507b..f34e06b4d8fa 100644
--- a/fs/udf/super.c
+++ b/fs/udf/super.c
@@ -739,6 +739,22 @@ static int udf_check_vsd(struct super_block *sb)
                 vsd = (struct volStructDesc *)(bh->b_data +
                                               (sector & (sb->s_blocksize - 1)));
                 nsr = identify_vsd(vsd);
+                /* Found NSR or end? */
+                if (nsr) {
+                        brelse(bh);
+                        break;
+                }
+                /*
+                 * Special handling for improperly formatted VRS (e.g., Win10)
+                 * where components are separated by 2048 bytes even though
+                 * sectors are 4K
+                 */
+                if (sb->s_blocksize == 4096) {
+                        nsr = identify_vsd(vsd + 1);
+                        /* Ignore unknown IDs... */
+                        if (nsr < 0)
+                                nsr = 0;
+                }
                 brelse(bh);
         }